There are known diagnostic devices for determining whether there is a malfunction in a voltage sensor installed in a system where a direct-current (DC) voltage output from a battery is converted by an inverter into an alternating-current (AC) voltage, and the AC voltage is supplied to an AC motor.
One of these diagnostic devices is disclosed in Japanese Patent Publication No. 4793058, which will be referred to as a conventional patent document.
The diagnostic device disclosed in the conventional patent document determines whether the absolute value of the difference between the battery voltage VBf measured by a battery voltage sensor and the inverter voltage VIf measured by an inverter input-voltage sensor is higher than a predetermined first value. Upon determining that the absolute value of the difference between the battery voltage VBf and the inverter voltage VIf is higher than the predetermined first value, the diagnostic device determines that there is a malfunction in one of the battery voltage sensor and the inverter input-voltage sensor.
The diagnostic device disclosed in the conventional patent document also obtains U-, V-, and W-phase currents flowing in an AC motor, and multiplies each of the U-, V-, and W-phase currents by a corresponding one of U-, V-, and W-phase command voltages. Then, the diagnostic device calculates, as a first output estimate, the sum of the product of the U-phase current and the U-phase command voltage, the product of the V-phase current and the V-phase command voltage, and the product of the W-phase current and the W-phase command voltage.
In addition, the diagnostic device disclosed in the conventional patent document calculates the product of request torque for the AC motor and the angular velocity of the AC motor to obtain a second output estimate accordingly.
The diagnostic device compares the first output estimate with the second output estimate to calculate the absolute value of the difference between the first and second output estimates. Then, the diagnostic device determines that there is a malfunction in the inverter input-voltage sensor when the absolute value of the difference between the first and second output estimates is higher than a predetermined second value.